This plugin should do the trick: http://wordpress.org/extend/plugins/wordpress-bootstrap-css/
It simply packages up boostrap's CSS and JS and inserts it in you theme (as long as the theme provides the right hooks to wordpress).
I created a theme for this purpose. More details: http://theme.firmasite.com/
The following is a tutorial based on underscore: WordPress Themes Templates Tutorial
It can easily be modified to work with Bootstrap. The only thing you have to pay attention to is the new column classes in Bootstrap 3 and not adding the tutorials CSSS near the end.
I created a theme the goal of integrating Twitter's bootstrap into Automattic's underscores with the smallest possible changeset.
I'm keeping it up to date rather frequently with both _s and Bootstrap. Here is the link:
At the time of writing, it's underscores' most popular fork
Other WordPress starter-themes that include Bootstrap: